Hemispherical detector

ABSTRACT

A hemispherical detector comprising a plurality of photodetectors arranged in a substantially contiguous array, the array being substantially in the shape of a half-sphere, the half-sphere defining a closed end and an open end, the open end defining a substantially circular face. Also provided is a method for constructing a hemispherical detector comprising the steps of making a press mold of the desired shape of the hemispherical detector, pouring a material into the press mold to form a cast, finishing the cast to remove any defects, coating the cast with a coating material, and attaching a plurality of photodetectors to the cast.

FIELD OF THE INVENTION

[0001] The present invention relates to the field of spectroscopicdetectors. Specifically, the present invention relates to ahemispherical detector for use with a transmittance or reflectancespectrometer which comprises a plurality of photodetectors.

BACKGROUND OF THE INVENTION

[0002] Infrared spectroscopy is a technique which is based upon thevibrations of the atoms of a molecule. In accordance with infraredspectroscopy, an infrared spectrum is generated by transmittingradiation through a sample and determining what portion of the incidentradiation is absorbed by the sample at a particular energy. Nearinfrared radiation is radiation having a wavelength between about 700 nmand about 2500 nm.

[0003] In general spectrometers (e.g., a spectrophotometer) can bedivided into two classes: transmittance spectrometers and reflectancespectrometers. In a transmittance spectrometer, light having a desirednarrow band of wavelengths is directed onto a sample, and a detectordetects the light which was transmitted through the sample. In contrast,in a reflectance spectrometer, light having a narrow band of wavelengthsis directed onto a sample and one or more detectors detect the lightwhich was reflected off of the sample. Depending upon its design, aspectrometer may, or may not, be used as both a transmittance and areflectance spectrometer.

[0004] A variety of different types of spectrometers are known in theart such as grating spectrometers, FT (Fourier transformation)spectrometers, Hadamard transformation spectrometers, AOTF (AcoustoOptic Tunable Filter) spectrometers, multiple discrete wavelength sourcespectrometers, filter-type spectrometers, scanning dispersivespectrometers, and double-beam spectrometers.

[0005] Filter-type spectrometers, for example, utilize a light source(such as a conventional light bulb) to illuminate a rotating opaquedisk, wherein the disk includes a number of narrow bandpass opticalfilters. The disk is then rotated so that each of the narrow bandpassfilters passes between the light source and the sample. An encoderindicates which optical filter is presently under the light source. Thefilters filter the light from the light source so that only a narrowselected wavelength range passes through the filter to the sample.Optical detectors are positioned to detect light which either isreflected by the sample (to obtain a reflectance spectra) or istransmitted through the sample (to generate a transmittance spectra).The amount of detected light is then measured, which provides anindication of the amount of absorbance of the light by the substanceunder analysis.

[0006] Multiple discrete wavelength source spectrometers use infraredemitting diodes (IREDs) as sources of near-infrared radiation. Aplurality (for example, eight) of IREDs are arranged over a sample worksurface to be illuminated for quantitative analysis. Near-infraredradiation emitted from each IRED impinges upon an accompanying opticalfilter. Each optical filter is a narrow bandpass filter which passes NIRradiation at a different wavelength. NIR radiation passing through thesample is detected by a detector (such as a silicon photodetector). Theamount of detected light is then measured, which provides an indicationof the amount of absorbance of the light by the substance underanalysis. IRED reflectance spectroscopy is also possible.

[0007] Acousto Optic Tunable Filter spectrometers utilize an RF signalto generate acoustic waves in a TeO₂ crystal. A light source transmits abeam of light through the crystal, and the interaction between thecrystal and the RF signal splits the beam of light into three beams: acenter beam of unaltered white light and two beams of monochromatic andorthogonally polarized light. A sample is placed in the path of one ofthe monochromatic beams and detectors are positioned to detect lightwhich either is reflected by the sample (to obtain a reflectancespectra) or is transmitted through the sample (to generate atransmittance spectra). The wavelength of the light source isincremented across a wavelength band of interest by varying the RFfrequency. The amount of detected light is then measured, which providesan indication of the amount of absorbance of the light by the substanceunder analysis.

[0008] In grating monochrometer spectrometers, a light source transmitsa beam of light through an entrance slit and onto a grating element (thedispersive element) to disperse the light beam into a plurality of beamsof different wavelengths (i.e., a dispersed spectrum). The dispersedlight is then reflected back through an exit slit on to a detector. Byselectively altering the path of the dispersed spectrum relative to theexit slit, the wavelength of the light directed to the detector can bevaried. The amount of detected light is then measured, which provides anindication of the amount of absorbance of the light by the substanceunder analysis. The width of the entrance and exit slits can be variedto compensate for any variation of the source energy with wavenumber.This approach lends itself to reflectance spectrometry.

[0009] Dual-beam spectrometers split radiation from a source into twobeams, half passing into a sample-cell compartment and the other halfinto a reference area. The reference beam then passes through anattenuator and on to a chopper, which is a motor-driven disk thatalternatively reflects the reference or transmits the beam to adetector. After dispersion by a prism or grating, the sample-cell beampasses to the sample and a detector detects the transmittance thatpasses through the sample or reflectance that reflects from the sample.If the two beams are identical in power, the detectors transmit similarelectrical signals to a null detector. The null detector in turnproduces an unfluctuating direct current. However, if the two beamsdiffer in power, the detectors transmit differing electrical signals tothe null detector. In this case, the null detector produces afluctuating electrical current, which is used to generate the spectraldata. For example, the fluctuating current can be used to drive asynchronous motor in one direction or the other depending upon the phaseof the current; with the synchronous motor mechanically linked to a pendrive of a recorder, which the synchronous motor causes to move togenerate the spectral data. This approach lends itself to bothtransmittance and reflectance spectrometry.

[0010] Detectors used in spectroscopy generally fall into two classes,photographic detectors, in which radiation impinges upon an unexposedphotographic film, and electronic detectors, in which the radiationimpinges upon a detector and is converted into an electrical signal.Electronic detectors provide the advantage of increased speed andaccuracy, as well as the ability to convert the spectral data into anelectronic format, which can be displayed, processed, and/or stored.Examples of electronic detectors include photomultiplier tubes andphotodetectors. Photomultiplier tubes are quite sensitive, but arerelatively large and expensive. Photodetectors provide the advantage ofreduced size and cost. These detectors include IR detectors, pin diodedetectors, charge coupled device detectors, and charge injection devicedetectors.

[0011] Conventionally, spectroscopic detectors are configured either asa single detector, flat detector, or a plurality of discrete detectorsarranged in common plane (e.g. a flat array). In either case, these“flat” detector arrangements inherently detect only a 3% portion of thetransmitted or reflected spectral data for 1 cm² detectors at a 2 cmdistance from the source detector.

[0012] As described in Burns & Ciurczak, HANDBOOK OF NEAR-INFRAREDANALYSIS, pp 42-43 (1992), detectors for measuring diffuse reflectanceare known which include either two or four opposing detectors arrangedat a 45 degree angle from the sample. In general, PbS detectors are usedfor measurements in the 1100-2500-nm region, whereas PbS “sandwiched”with silicon photodiodes are most often used for visible-near-infraredapplications (typically 400-2600 nm).

[0013] The signal from the detectors is added to a low-noise, high-gainamplifier and then converted from analog to digital. The digital signalis exported from the instrument to an on-board or external microcomputerfor data processing, calibration, and storage. The computer records asignal representing the actual wave-length used for measurement with theraw reflectance or transmittance digital data. This function is repeatedfor both the sample and the reference. The spectrum, then, is thedifference between the raw reflectance measurement of the sample and theraw reflectance measurement of the reference material. Raw reflectanceis converted to absorbance using the function Absorbance=−log(10)*Reflectance, commonly referred to as log 1/R. Raw transmittance isconverted to absorbance using the expression log 1/T.

SUMMARY OF THE INVENTION

[0014] When configured with four opposing 1 cm² detectors at 45 degreeangles and 2 cm from the sample, the diffuse reflectance detectordescribed above provides the advantage of collecting spectral data fromfour different vantage points, as compared to more conventional “flat”detector arrangements. However, even with this configuration, only about12% of the reflected spectral data is detected. Moreover, thisconfiguration is not suitable for use with a transmittance spectrometer.

[0015] In accordance with a first embodiment of the present invention, ahemispherical detector for use with a transmittance spectrometer isprovided which comprises a plurality of photodetectors arranged in asubstantially contiguous array, the array being substantially in theshape of a half-sphere, the half-sphere defining a closed end and anopen end, the open end defining a substantially circular face. In use, asample to be analyzed preferably intersects a plane passing through thesubstantially circular face, and a transmitted beam of light from thetransmittance spectrometer intersects the plane at a 90 degree angle to,and at a center-point of, said substantially circular face. In thismanner, substantially all of the light which passes through the sampleis detected by the detector array. Currently, most photodetectors have aflat surface. Therefore, the individual photodetectors which comprisethe array of photodetectors are preferably about 0.5-3 mm² in order toprovide a substantially spherical shape. If available, smallerphotodetectors can also be used. In this manner, except for beams oflight which strike between photodetectors, all of the light which passesthrough the sample is detected by the photodetector array. In thisregard, it is believed that this configuration can detect about 80% ofthe spectral data.

[0016] In accordance with a second embodiment of the present invention,a hemispherical detector for use with a reflectance spectrometer isprovided which comprises a plurality of photodetectors arranged in asubstantially contiguous array, the array being substantially in theshape of a truncated half-sphere, the truncated half-sphere defining afirst open end and a second open end, the second open end defining asubstantially circular face having a diameter (“d”), the first open endhaving a cutout formed therein, wherein the cutout defines an area whichis less than Π(d/2)². In use, a sample to be analyzed preferablyintersects a plane passing through the substantially circular face, anda transmitted beam of light from the reflectance spectrometer passesthrough the second open end in a direction perpendicular to the planeand co-axial with a center-point of said substantially circular face. Inthis manner, substantially all of the light which reflects off of thesample is detected by the detector array. As with the first embodimentdescribed above, the individual photodetectors which comprise the arrayof photodetectors are preferably about 0.5-3 mm² in order to provide asubstantially spherical shape. In this manner, except for beams of lightwhich strike between photodetectors, or are reflected back through thefirst open end, all of the light which is reflected off of the sample isdetected by the photodetector array. Preferably, the area of the openingdefined by the first open end is minimized in order to maximize thepercentage of the reflected light which is received by the detectorarrays. However, the opening must remain sufficiently large to allow thetransmitted beam of light to impinge upon the sample. Also, motion dueto the operation of the spectrometer may cause the shell of the detectorto infringe the path of the beam of light. Most preferably, the firstopen end is a circular cut-out having a diameter of approximately 5 mm.

[0017] In accordance with a third embodiment of the present invention, ahemispherical detector for use with a reflectance or transmittancespectrometer is provided which comprises a plurality of photodetectorsarranged in a substantially contiguous array, the array beingsubstantially in the shape of a half-sphere. The half-sphere includes afirst portion and a second portion. The first portion is in the shape ofa truncated half sphere, the truncated half sphere defining a first openend and a second open end, the second open end defining a substantiallycircular face having a diameter (“d”), the first open end having acutout formed therein, wherein the cutout defines an area which is lessthan Π(d/2)². The second portion is removably secured to the first openend. When performing a transmittance measurement, the second portion issecured to the first portion, thereby forming photodetector array whichis substantially in the shape of a half-sphere. The hemisphericaldetector can then be used in the manner described above with referenceto the first embodiment. In order to perform a reflectance measurement,the second portion is removed from the first portion, thereby formingphotodetector array which is substantially in the shape of a truncatedhalf-sphere. The hemispherical detector can then be used in the mannerdescribed above with reference to the second embodiment.

[0018] It is believed that the detectors of the second and thirdembodiment, like the first embodiment, can detect approximately 80% ofthe spectral data.

[0019] In each of the embodiments described above, openings arepreferably provided in the shell to allow wires to contact thephotodetectors. This prevents the wires from interfering with dataacquisition.

[0020] The detectors in accordance with the present invention may beused in a variety of spectrometers including, for example, filter-typespectrometers, multiple discrete wavelength source spectrometers, AOTF(Acousto Optic Tunable Filter) spectrometers, grating spectrometers, FT(Fourier transformation) spectrometers, Hadamard transformationspectrometers, post-dispersive monochrometer spectrometers, double beamspectrometers, and scanning dispersive spectrometers. In theseembodiments the detectors provide the advantage of a more accuratemeasurement by increasing the percentage of spectral data which isdetected.

[0021] In the embodiments described above, the substantially circularface preferably has a diameter of from about 1.5 mm to about 1 m.

[0022] The hemispherical detectors in accordance with the presentinvention may be constructed in a number of ways.

[0023] For example, the hemispherical detector may be constructed by amold method. In accordance with this method, a press mold is created, amaterial is poured into the mold to create a cast (which forms the shellof the detector) and a plurality of photodetectors are attached to thecast. This has the advantage of quick and efficient construction.Moreover, a plurality of uniform hemispherical detectors may be made.The cast preferably has a diameter of from about 1.5 mm to about 1 m.

[0024] The hemispherical detector may also be constructed by an airformmethod. In accordance with this method, a malleable airform, e.g.,plastic, may be fabricated to the proper shape and size, inflated, andthen coated with a hardening material to create the shell of thedetector. This has the advantage of a strong and stable hemisphericaldetector at a marginal cost. Also, this method provides the advantagethat detectors of differing sizes can easily be constructed by modifyingthe amount of material in the airform. The malleable airform preferablyhas a diameter of from about 1.5 mm to about 1 m.

[0025] The hemispherical detector may also be constructed by geodesicdome method. In accordance with this method, a plurality of pentagons,hexagons, and half hexagons are joined together in a geodesic domeshape, e.g., such that every pentagon is surrounded by 5 hexagons,half-hexagons, or combination thereof. Photodetectors or fillings withphotodetectors attached may be secured in the areas between the struts.This has the advantage of a versatile and sturdy construction. Thegeodesic dome shape preferably has a diameter of from about 1.5 mm to 1m. Moreover, as six struts could form the entire circumferential lengthof the dome, each strut preferably has a length of from about 0.39 mm to0.26 m.

[0026] Preferably, the ceramic mold, airform, or geodesic domehemispherical detector construction method may be further modified toallow for additional wiring. In this regard, apertures may be drilled inthe hemispherical detector to allow wiring to contact the photodetector.

[0027] Although the above-referenced methods of construction arepreferred, other methods known in the art may alternatively be used.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1(a) illustrates a common instrument design for atransmittance spectrometers.

[0029]FIG. 1(b) illustrates a common instrument design for a reflectancespectrometer.

[0030]FIG. 2(a) is a side perspective view of a detector in accordancewith a first embodiment of the present invention which is suitable foruse with a transmittance spectrometer, the detector being in the shapeof a half sphere.

[0031]FIG. 2(b) is a side view of the detector of FIG. 2(a), illustratedas a cross-section through a line A-A, which bisects the half sphere.

[0032]FIG. 2(c) is a side view of the detector of FIG. 2(a).

[0033]FIG. 2(d) is a front view of the detector of FIG. 2(a).

[0034]FIG. 3(a) is a side perspective view of a detector in accordancewith a second embodiment of the present invention which is suitable foruse with a reflectance spectrometer, the detector being in the shape ofa truncated half sphere.

[0035]FIG. 3(b) is a side view of the detector of FIG. 3(a), illustratedas a cross-section through a line A-A, which bisects the truncated halfsphere.

[0036]FIG. 3(c) is a side-view of the detector of FIG. 3(a).

[0037]FIG. 3(d) is a front view of the detector of FIG. 3(a).

[0038]FIG. 3(e) is a rear perspective view of the detector of FIG. 3(a).

[0039]FIG. 4(a) is a side perspective view of a detector in accordancewith a third embodiment of the present invention which is suitable foruse with both a transmittance and a reflectance spectrometer, thedetector being in the shape of a half sphere.

[0040]FIG. 4(b) is a side view of the detector of FIG. 4(a), illustratedas a cross-section through a line A-A, which bisects the half sphere.

[0041]FIG. 4(c) is a side view of the detector of FIG. 4(a).

[0042]FIG. 4(d) is a front view of the detector of FIG. 4(a).

[0043]FIG. 4(e) is an exploded rear perspective view of the detector ofFIG. 4(a) showing the first and second portions disengaged.

[0044] FIGS. 4(f) and 4(g) illustrate the relative dimensions of ahemispherical detector which is substantially in the shape of a halfsphere or substantially in the shape of a truncated half sphere.

[0045]FIG. 5 is a schematic representation of a filter-typespectrometer.

[0046]FIG. 6 shows a schematic representation of a rotating tiltingfilter wheel utilizing wedge interference filters having a lightblocking flag.

[0047]FIG. 7 shows a schematic representation of a spinning filtersystem in which the light passes through an encoder wheel.

[0048]FIG. 8 shows a schematic representation of a typicalpre-dispersive monochrometer-based instrument.

[0049]FIG. 9 shows a schematic representation of a post-dispersivemonochrometer-based instrument.

[0050]FIG. 10 is a schematic diagram of a Multiple discrete wavelengthsource spectrometer which uses infrared emitting diodes (IREDs) as asource of near-infrared radiation.

[0051]FIG. 11 shows a schematic diagram of an Acousto Optic TunableFilter spectrometer.

[0052]FIG. 12 is a schematic representation of a Double-BeamSpectrometer.

[0053]FIG. 13 is a schematic diagram of a ceramic mold method ofconstruction of the detector.

[0054]FIG. 14 is a schematic diagram of a malleable airform method ofconstruction of the detector.

[0055]FIG. 15 is a schematic diagram of geodesic dome method ofconstruction of the detector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0056] FIGS. 1(a-b) show the two most prevalent basic instrument designscommon in modem near-infrared analysis; a transmittance spectrometer andreflectance spectrometer.

[0057]FIG. 1(a) shows a transmittance spectrophotometer and FIG. 1(b)shows a reflectance spectrometer. In both cases, a monochrometer 2produces a light beam 5 having desired narrow band of wavelengths fromlight 8 emitted from a light source 1, and the light beam 5 is directedonto a sample 3. However, in the case of a transmittance spectrometer, aplurality of detectors 4 are positioned to detect the light 6 which istransmitted through the sample 3, and in the case of a reflectancespectrometer, the plurality of detectors 4 are positioned to detect areflected light beam 7 which is reflected off the sample 3. Dependingupon its design, a spectrometer may, or may not, be used as both atransmittance and a reflectance spectrometer.

[0058] Reflectance measurements penetrate only 1-4 mm of the frontsurface of ground samples. This small penetration of energy into asample brings about greater variation when measuring nonhomogeneoussamples than transmittance techniques.

[0059] In transmittance measurements, the entire path length of thesample is integrated into the spectral measurement, thereby reducingerrors due to non-homogeneity of samples. Transmittance techniques aremost useful for measuring large particles. For fine particles, the frontsurface scatter brings about a loss of energy transmitted through asample, with the net effect being a decrease in the signal-to-noise ofthe instrument. In transmittance, higher frequency energy is mostcommonly used due to its greater depth of penetration into the sample.The higher frequency energy (800-1400 nm) is more susceptible to frontsurface scattering than lower frequency energy.

[0060] Transmittance measurements should therefore be optimized takinginto consideration the relationships between the frequency used formeasurement, front surface scatter, and the path length of the sample.In transmittance measurements, particle size can be small enough tobegin to scatter most of the energy striking the sample. If the particlesize is sufficiently small, the instrument will not transmit enoughenergy through the sample for the detectors to record a signal. Tocompensate, a preferred spectrophotometer would have both transmittanceand reflectance capabilities.

[0061] FIGS. 2(a-d) shows a hermispherical detector in accordance with afirst embodiment of the present invention for use with a transmittancespectrometer. The detector includes a plurality of photodetectors 12arranged in a substantially contiguous array. The photodetectors may,for example, be InAs photon detectors, InSb photon detectors, PbS photondetectors, PbSe photon detectors, or InGaAs photon detectors. Theparticular photodetector used is dictated by the anticipatedapplication. For example, InAs, InSb, PbS, and PbSe photon detectors aregenerally used for infrared applications, whereas InGaAs, PbS, and InAsphoton detectors are generally used for near-infrared applications. Thephotodetectors may also be, for example, photconductive photondetectors; PbSi photoconductive photon detectors; photvoltaic photondetectors; photodiodes; Si, Thermoelectrically-cooled Si, GaP, GaAsP, orInGaAs photodiodes; PbS detectors sandwiched with Si photodiodes;photconductive cells; Cds or PbSe/PbS photconductive cells; andInAs/InSb photovoltaic detectors. Ge photodetectors may also be used.The array may be comprised of a single type of photodetector, or,alternatively, two or more different types of photodetectors may beused. Moreover, the array may include photodetector clusters (i.e., twoor more different types of photodetectors fabricated as a single unit orcluster).

[0062] The photodetectors 12 arranged in a substantially contiguousarray are supported on a shell 10 which has an inner surface 11, whichis configured to hold the photodetectors 12 arranged in a substantiallycontiguous array substantially in the shape of a half sphere. Thehalf-sphere defines a closed end 50 and an open end 60. The open end 60defines a substantially circular face 70. In use, a sample to beanalyzed preferably intersects a plane 80 passing through thesubstantially circular face 70, and the transmitted beam of light 5 fromthe transmittance spectrometer (FIG. 1A) intersects the plane at a 90degree angle, and at a center-point 90 of said substantially circularface 70. In this manner, substantially all of the light which passesthrough the sample is detected by the photodetectors 12 arranged in asubstantially contiguous array. Currently, photodetectors have a flatsurface. Therefore, the individual photodetectors which comprise thearray of photodetectors are preferably about 0.5-3 mm² in order toprovide a substantially spherical shape. In this manner, except forbeams of light which strike between photodetectors, all of the lightwhich passes through the sample is detected by the photodetector array.

[0063] Openings 51 are provided in the shell 10 to allow wires 53 tocontact the photodetectors 12. This prevents the wires 53 frominterfering with data acquisition.

[0064] FIGS. 3(a-e) show a hemispherical detector for use with areflectance spectrometer, with similar components bearing like referencenumerals to FIGS. 2(a-c). The detector includes the plurality ofphotodetectors 12′ arranged in a substantially contiguous array. Theplurality of photodetectors 12′ arranged in a substantially contiguousarray are supported on a shell 10′ which has an inner surface 11′ whichis configured to hold the photodetectors 12′ arranged in a substantiallycontiguous array substantially in the shape of a truncated half-sphere.The truncated half-sphere defines a first open end 110 and a second openend 60′. The second open end 60′ defines the substantially circular face70′ as having a diameter (“d”). The first open end 110 has a cutout 111formed therein, wherein the cutout defines an area which is less thanΠ(d/2)². In use, a sample to be analyzed preferably intersects the plane80′ passing through the substantially circular face 70′, and atransmitted beam of light 5′ from the reflectance spectrometer (FIG. 1b)passes through the second open end 60′ in a direction perpendicular tothe plane 80′ and co-axial with a center-point 90′ of said substantiallycircular face 70′. In this manner, substantially all of the light whichreflects off of the sample is detected by the plurality ofphotodetectors 12′ arranged in a substantially contiguous array. As withthe first embodiment described above, the individual photodetectorswhich comprise the array of photodetectors are preferably about 0.5-3mm² in order to provide a substantially spherical shape. In this manner,except for beams of light which strike between photodetectors, or arereflected back through the first open end, all of the light which isreflected off of the sample is detected by the photodetector array.Preferably, the area of the cut-out 111 is minimized in order tomaximize the percentage of the reflected light which is received by thedetector array. The cut-out should, however, be large enough to allowthe beam of lights to reach the sample. Most preferably, the cutout 111is a circular cut-out having a diameter of approximately 5 mm.

[0065] Openings 51′ are provided in the shell 10′ in order to allowwires 53′ to contact the photodetectors 12′.

[0066] FIGS. 4(a-e) show a hemispherical detector for use with areflectance or transmittance spectrometer, with similar componentsbearing like reference numerals to FIGS. 2(a-d). The detector includesthe plurality of photodetectors 12″ arranged in a substantiallycontiguous array. The photodetectors 12″ arranged in a substantiallycontiguous array are supported on a two part shell 10″ which has aninner surface 11″ which is configured to hold the plurality ofphotodetectors 12″ arranged in a substantially contiguous arraysubstantially in the shape of a half-sphere. The shell 10″ includes afirst portion 200 and a second portion 205. The first portion 200 is inthe shape of a truncated half sphere. The truncated half sphere definesa first open end 110′ and a second open end 60″, the second open end 60″defining a substantially circular face having a diameter (“d”). Thefirst open end 110′ has a cutout 111′ formed therein, wherein the cutoutdefines an area which is less than Π(d/2)². As shown in FIG. 4(e), thesecond portion 205 is removably secured to the first open end 110′. Inthis regard, the second portion 205 could be removably secured to thesecond open end: via a friction fit; by providing respective threads onthe first and second portion; utilizing a latch mechanism; or in anyother manner known to one of skill in the art.

[0067] When performing a transmittance measurement, the second portion205 is secured to the first portion 200, thereby forming photodetectorarray which is substantially in the shape of a half-sphere. Thehemispherical detector can then be used in the manner described abovewith reference to the FIGS. 2(a-d). In order to perform a reflectancemeasurement, the second portion 205 is removed from the first portion200, thereby forming the photodetector 12″ arranged in a substantiallycontiguous array which is substantially in the shape of a truncatedhalf-sphere. The hemispherical detector can then be used in the mannerdescribed above with reference to FIGS. 3(a-e).

[0068] Openings are 51″ are provided in the shell 10″ in order to allowwires 53″ to contact the photodetectors 12″.

[0069] As set forth above, the hemispherical detectors in accordancewith FIGS. 2, 3, and 4 include photodetectors arranges in an array whichis substantially in the shape of sphere or half sphere. Referring toFIGS. 4(f) and 4(g), in accordance with the present invention, a arrayis considered substantially in the shape of a half sphere or truncatedhalf sphere: i) if a ratio of a minimum radius (Amin) and a maximumradius (Amax) of the face 70 of the sphere or truncated sphere (from thecenterpoint 90) is equal to Amin/Amax=1+/−0.1; and if the distance bfrom the centerpoint 90 on the face 70 to any point on the array isequal to b=(Amin+Amax)/2+/−0.1*((Amin+Amax)/2). It should be noted,moreover that although the shell 50 is illustrated as being in the shapeof a sphere or half-sphere, the shell 50 can, of course, have adifferent shape provided that the photodetector array is substantiallyin the shape of a half sphere or truncated half sphere.

[0070] The detectors in accordance with the present invention may beused with a variety of spectrometers including, for example, filter-typespectrometers, multiple discrete wavelength source spectrometers, AOTF(Acousto Optic Tunable Filter) spectrometers, grating spectrometers, FT(Fourier transformation) spectrometers, Hadamard transformationspectrometers, post-dispersive monochrometer spectrometers, double beamspectrometers, and scanning dispersive spectrometers.

[0071] The wires 53 shown in FIGS. 2, 3 and 4 may be connected to one ormore data buses in order to facilitate processing of the data acquired.

[0072]FIG. 5 shows a monochromatic filter-type spectrometer 501, whichutilizes a light source 502, such as the conventional light bulb shownin the figure to illuminate a rotating opaque circular disk 504, whereinthe disk includes a number of narrow bandpass optical filters 507. Thedisk can be rotated so that each of the narrow bandpass filters passesbetween the light source and a sample 509. An encoder 511 controls whichoptical filter is presently under the light source. The filters 507filter the light from the light source 502 so that only a narrowselected wavelength range passes through the filter to the sample 3. Asillustrated in FIG. 5, the filter-type spectrometer 501 may be used withany one of the detectors described above in FIGS. 2-4.

[0073]FIGS. 6 and 7 illustrate two basic forms of filter-type NIRspectrophotometers utilizing a tilting filter concept.

[0074]FIG. 6 shows a rotating tilting filter wheel utilizing wedgeinterference filters having a light blocking flag. Light is transmittedfrom the light source 502 through the filter wheel 600 at varyingwavelengths and bandpasses which is dependent on the incident angle ofthe light passing through the interference filter wedge to the sample.

[0075]FIG. 7 shows a spinning filter system in which the light passesthrough an encoder wheel 700, having a plurality of interference filters701, to the sample 3. The spinning filter system operates using the samebasic principle as the tilting filter of FIG. 6, but the interferencefilters 701 of the spinning filter system are mounted in an encoderwheel 700 for greater positioning accuracy (wavelength reproducibility)and greater reliability. As illustrated in FIGS. 6 and 7, the rotatabletilting filter wheel and spinning filter system may be used with any oneof the detectors described above in FIGS. 2-4.

[0076]FIG. 8 shows a typical pre-dispersive monochrometer-basedinstrument where the light is dispersed prior to striking the sample.Referring to FIG. 8, the light source 502 transmits a beam of lightthrough an entrance slit 800 and onto a grating 810. The grating 810separates the light into a plurality of beams of different wavelengths.Via the order sorting 820 (to eliminate undesired wavelengths) and stds830 (to provide a wavelength standard for calibration) components, adesired band of wavelengths is selected for transmission to the sample3. As illustrated, this spectrometer may also be used with any one ofthe detectors described above in FIGS. 2-4.

[0077]FIG. 9 shows a typical post-dispersive monochrometer. This type ofinstrument provides the advantage of allowing the transmission of moreenergy on the sample via either a single fiberoptic strand or afiberoptic bundle. Referring to FIG. 9, white light is transmittedthrough the fiberoptic strand or fiberoptic bundle 900 and onto thesample 3. The light is then reflected off of the sample 3 and back tothe grating 910 (the dispersive element). After striking the grating 910the light is separated into the various wavelengths prior to striking adetector. The post-dispersive monochrometer can be used with thereflectance detectors of FIGS. 2(a-d) or of FIGS. 4(a-e) (with the firstportion 200 secured to the second portion 205). It should be noted thatin this case, the detectors detect a reflectance spectrum rather than atransmittance spectrum.

[0078]FIG. 10 is a diagram of a Multiple discrete wavelength sourceSpectrometer 17, which uses infrared emitting diodes (IREDs) as a sourceof near-infrared radiation. A plurality (for example, eight) of IREDs 19are arranged over a sample work surface to be illuminated forquantitative analysis. Near-infrared radiation emitted from each IREDimpinges upon an accompanying optical filter 23. Each optical filter isa narrow bandpass filter which passes NIR radiation at a differentwavelength. As illustrated, this spectrometer may be used with any oneof the detectors described above in FIGS. 2-4.

[0079]FIG. 11 depicts an Acousto Optic Tunable Filter spectrometerutilizing an RF signal 1229 to generate acoustic waves in a TeO₂ crystal1232. A light source 1230 transmits a beam of light through the crystal1232, and the crystal splits the beam of light into three beams: acenter beam of unaltered white light 1237 and two beams of monochromaticand orthogonally polarized light 1240. A sample 1242 is placed in thepath of one of the monochromatic beams. The wavelength of themonochromatic light can be incremented across a wavelength band ofinterest by varying the RF frequency. As illustrated, this spectrometermay also be used with any one of the detectors described above in FIGS.2-4.

[0080]FIG. 12 depicts a Double Beam spectrometer. The Double BeamSpectrometer uses a light emitting source 1500, e.g., a tungsten lamp,to produce a beam 1502 of light which is split into two parts by a beamsplitting device 1504, e.g., a half-silvered mirror, after passingthrough a filter device 1516. The first beam half 1506 passes through asample 1508, which is placed in the path of one of the beams. The secondbeam half passes through a reference 1510. Detectors 1512, which may beany of the detectors described above in FIGS. 2-4, detect the radiationeither transmitted through or reflected from the sample 1508 andreference 1510. Next, a null detector 1514 compares the results fromboth detectors and sends an electrical code corresponding to whether theresults from both detectors are the same or different. This allows achart corresponding to different wavelengths to be constructed.

[0081]FIG. 13 depicts a Mold method of constructing a hemisphericaldetector in accordance with the present invention. A press mold 1650 ismade to resemble the desired shape of the hemispherical detector. Amaterial 1660, e.g., plastic, that is a liquid at a high temperature anda solid at a lower temperature is poured into the press mold 1650 toform a cast 1652. After the mold has set, the cast 1652 is removed fromthe press mold 1650. Next, the cast 1652 is finished, e.g., sanded, toremove any defects caused by the process and then coated by a coatingmaterial 1654 designed to preserve the material. A plurality ofphotdetectors 1656 are then attached to the inner surface 1658 of thecast 1652 so as to form a substantially contiguous array. Apertures 1670may be drilled through the cast 1652, in order to facilitate theattachment of wires 1658 to the photodetectors 1656. Alternatively, themold 1650 itself could be configured to provide the apertures 1670.

[0082]FIG. 14 depicts an Airform method for constructing a hemisphericaldetector in accordance with the present invention. A malleable airform2100, e.g., plastic, is fabricated to the proper shape and size. Themalleable airform 2100 is placed on a ring base 2102 and inflated with ablower fan 2104. A hardening material 2106, e.g., polyurethane foam, isthen applied to the interior surface of the malleable airform 2100 tostabilize the hemispherical structure. After the hardening material 2106has set, a plurality of reinforcing bars 2107, e.g., plastic, may beattached to the hardening material 2106 in order to stabilize thehardening material 2106. A second layer of hardening material 2108 maybe applied to the interior surface 2110 of the reinforcing bars 2107 inorder to further stabilize it. After the second of hardening material2108 has hardened, a plurality of photodetectors 1656 arranged in asubstantially contiguous array are attached to the second layer ofhardening material 2106 to form the hemispherical detector. Apertures1670 may be drilled through the hardening material 2106, in order tofacilitate the attachment of wires 1658 to the photodetectors 1656. FIG.15 depicts a geodesic dome method for constructing a hemisphericaldetector in accordance with the present invention. A plurality ofpentagons 3000, hexagons 3002, and half hexagons 3004 are assembled froma plurality of sufficiently sturdy small struts 3006, e.g., hardenedplastic. Alternatively, the pentagons 3000, hexagons 3002, andhalf-hexagons 3004 could be pre-formed (e.g., molded plastic). In anyevent, the pentagons 3000, hexagons 3002, and half hexagons 3004 arethen joined together in a geodesic dome shape, e.g., such that everypentagon is surrounded by 5 hexagons 3002, half-hexagons 3004, orcombination thereof. A plurality of fitted photodetectors 3056 shaped tofit in a plurality of areas 3012 between the small struts 3006 may thenbe secured to said areas 3012 by a mechanical, e.g., screws, or astatic, e.g., glue, material so as to form a substantially contiguousarray. Another option is to fit a plurality of fillings 3014 shaped tofit in the areas 3012 between the small struts 3006 and then secure thefillings 3014 to the areas by a mechanical, e.g., screws, ornon-mechanical, e.g., glue, material. Photodetectors 1656 may then beattached in a substantially contiguous array to the fillings by amechanical, e.g., screws, or static, e.g., glue, material to form ahemispherical detector. Also, a plurality of fitted photodetectors 3056may fill a portion of the areas, as detailed above, and then theremaining areas may be filled with a plurality of fillings 3014, alsodetailed above, which may then have photodetectors attached, so as toform a substantially contiguous array. Apertures 1670 may be drilledthrough the fillings 3014, in order to facilitate the attachment ofwires 1658 to the photodetectors 1656. Alternatively, the wires 1658 maybe attached directly to the back of the fitted photdetectors 3056.

What is claimed is:
 1. A detector comprising: a plurality ofphotodetectors arranged in a substantially contiguous array, the arraybeing substantially in the shape of a half-sphere, the half-spheredefining a closed end and an open end, the open end defining asubstantially circular face.
 2. A detector comprising a plurality ofphotodetectors arranged in a substantially contiguous array, the arraybeing substantially in the shape of a truncated half-sphere, thetruncated half-sphere defining a first open end and a second open end,the second open end defining a substantially circular face having adiameter (“d”), the first open end having a cutout formed therein,wherein the cutout defines an area which is less than Π(d/2)².
 3. Adetector comprising a plurality of photodetectors arranged in asubstantially contiguous array, the array being substantially in theshape of a half-sphere, the half-sphere includes a first portion and asecond portion, the first portion being in the shape of a truncated halfsphere, the truncated half sphere defining a first open end and a secondopen end, the second open end defining a substantially circular facehaving a diameter (“d”), the first open end having a cutout formedtherein, wherein the cutout defines an area which is less than Π(d/2)²,the second portion being removably secured to the first open end, thesecond portion covering the cutout when the second portion is secured tothe first open end.
 4. A detector according to claim 1 furthercomprising a filter-type spectrometer which utilizes a light source suchto illuminate a rotating opaque disk.
 5. A detector according to claim 2further comprising a filter-type spectrometer which utilizes a lightsource such to illuminate a rotating opaque disk.
 6. A detectoraccording to claim 3 further comprising a filter-type spectrometer whichutilizes a light source such to illuminate a rotating opaque disk.
 7. Adetector according to claim 1 further comprising a near IR spectrometerthat utilizes a tilting filter wheel.
 8. A detector according to claim 2further comprising a near IR spectrometer that utilizes a tilting filterwheel.
 9. A detector according to claim 3 further comprising a near IRspectrometer that utilizes a tilting filter wheel.
 10. A detectoraccording to claim 1 further comprising a near IR spectrometer utilizinginterference filters mounted in an encoder wheel.
 11. A detectoraccording to claim 2 further comprising a near IR spectrometer utilizinginterference filters mounted in an encoder wheel.
 12. A detectoraccording to claim 3 further comprising a near IR spectrometer utilizinginterference filters mounted in an encoder wheel.
 13. A detectoraccording to claim 1 further comprising a pre-dispersivemonochrometer-based instrument where the light is dispersed prior tostriking the sample.
 14. A detector according to claim 2 furthercomprising a pre-dispersive monochrometer-based instrument where thelight is dispersed prior to striking the sample.
 15. A detectoraccording to claim 3 further comprising a pre-dispersivemonochrometer-based instrument where the light is dispersed prior tostriking the sample.
 16. A detector according to claim 1 furthercomprising a post-dispersive monochrometer using a fiberoptic strand orbundle.
 17. A detector according to claim 2 further comprising apost-dispersive monochrometer using a fiberoptic strand or bundle.
 18. Adetector according to claim 3 further comprising a post-dispersivemonochrometer using a fiberoptic strand or bundle.
 19. A detectoraccording to claim 1 further comprising a multiple discrete wavelengthsource spectrometer utilizing infrared emitting diodes as a source ofnear-infrared radiation.
 20. A detector according to claim 2 furthercomprising a multiple discrete wavelength source spectrometer utilizinginfrared emitting diodes as a source of near-infrared radiation.
 21. Adetector according to claim 3 further comprising a multiple discretewavelength source spectrometer utilizing infrared emitting diodes as asource of near-infrared radiation.
 22. A detector according to claim 1further comprising an Acousto Optic Tunable Filter spectrometerutilizing an RF signal to generate acoustic waves in a TeO₂ crystal. 23.A detector according to claim 2 further comprising an Acousto OpticTunable Filter spectrometer utilizing an RF signal to generate acousticwaves in a TeO₂ crystal.
 24. A detector according to claim 3 furthercomprising an Acousto Optic Tunable Filter spectrometer utilizing an RFsignal to generate acoustic waves in a TeO₂ crystal.
 25. A detectoraccording to claim 1 further comprising a double beam spectrometerutilizing a beam splitting device.
 26. A detector according to claim 2further comprising a double beam spectrometer utilizing a beam splittingdevice.
 27. A detector according to claim 3 further comprising a doublebeam spectrometer utilizing a beam splitting device.
 28. A method forconstructing a detector comprising the steps of: making a press mold ofthe desired shape of the detector; pouring a material into the pressmold to form a cast; and attaching a plurality of photodetectors to thecast so as to form a substantially contiguous array which issubstantially in the shape of a half-sphere or a truncated half-sphere.29. A method for constructing a detector as recited in claim 28 furthercomprising the step of finishing the cast to remove any defects.
 30. Amethod for constructing a detector as recited in claim 29 furthercomprising the step of coating the cast with a coating material.
 31. Amethod for constructing a detector comprising the steps of: fabricatinga malleable airform in a desired shape of a detector; placing themalleable airform on a ring base; applying a hardening material to theinterior surface of the malleable airform; attaching a plurality ofreinforcing bars in order to stabilize the hardening material; adding asecond layer of hardening material to the interior surface of thereinforcing bars; and attaching a plurality of photodetectors to thesecond layer so as to form a substantially contiguous array which issubstantially in the shape of a half-sphere or a truncated half-sphere.32. A method for constructing a detector comprising the steps of:joining a plurality of pentagons, hexagons, and half hexagons to form ageodesic dome; and securing a plurality of fitted photodetectors to thegeodesic dome so as to form a substantially contiguous array.
 33. Amethod for constructing a detector as recited in claim 32 furthercomprising the step of assembling the plurality of pentagons, hexagons,and half hexagons from a plurality of struts.
 34. The method forconstructing a detector as recited in claim 33 further comprising thestep of securing a plurality of fillings to the struts.
 35. The methodfor constructing a detector as recited in claim 34 further comprisingattaching a plurality of photdetectors to the fillings so as to form thesubstantially contiguous array.
 36. The detector as recited in claim 1wherein the photodetectors are photoconductive photon detectors.
 37. Thedetector as recited in claim 3.6 wherein the photoconductive photondetectors are selected from the group consisting of PbSi photoconductivephoton detectors, PbSe photon detectors, InAs photon detectors, andInGaAs photon detectors.
 38. The detector as recited in claim 2 whereinthe photodetectors are photoconductive photon detectors.
 39. Thedetector as recited in claim 38 wherein the photoconductive photondetectors are selected from the group consisting of PbSi photoconductivephoton detectors, PbSe photon detectors, InAs photon detectors, andInGaAs photon detectors
 40. The detector as recited in claim 3 whereinthe photodetectors are photoconductive photon detectors.
 41. Thedetector as recited in claim 40 wherein the photoconductive photondetectors are selected from the group consisting of PbSi photoconductivephoton detectors, PbSe photon detectors, InAs photon detectors, andInGaAs photon detectors
 42. The detector as recited, in claim 1 whereinthe photodetectors are selected from the group consisting ofphotovoltaic photon detectors, InSb photon detectors, photodiodes,photoconductive cells, and HgCdTe photoconductive detectors.
 43. Thedetector as recited in claim 2 wherein the photodetectors are selectedfrom the group consisting of photovoltaic photon detectors, InSb photondetectors, photodiodes, photoconductive cells, and HgCdTephotoconductive detectors.
 44. The detector as recited in claim 3wherein the photodetectors are selected from the group consisting ofphotovoltaic photon detectors, InSb photon detectors, photodiodes,photoconductive cells, and HgCdTe photoconductive detectors.
 45. Thedetector as recited in claim 1 wherein the photodetectors are selectedfrom the group consisting of Ge detectors, Si detectors, and PbSdetectors.
 46. The detector as recited in claim 2 wherein thephotodetectors are selected from the group consisting of Ge detectors,Si detectors, and PbS detectors.
 47. The detector as recited in claim 3wherein the photodetectors are selected from the group consisting of Gedetectors, Si detectors, and PbS detectors.
 48. The detector as recitedin claim 1 wherein the substantially circular face has a diameter offrom about 1.5 mm to about 1 m.
 49. The detector as recited in claim 2wherein the substantially circular face has a diameter of from about 1.5mm to about 1 m.
 50. The detector as recited in claim 3 wherein thesubstantially circular face has a diameter of from about 1.5 mm to about1 m.
 51. The method for constructing a detector as recited in claim 28wherein the cast has a diameter of from about 1.5 mm to about 1 m. 52.The method for constructing a detector as recited in claim 31 whereinthe malleable airform has a diameter of from about 1.5 mm to about 1 m.53. The method for constructing a detector as recited in claim 33wherein the geodesic dome shape has a diameter of from about 1.5 mm toabout 1 m.
 54. The method for constructing a detector as recited inclaim 33 wherein a strut further has a length of from about 0.39 mm toabout 0.26 m.
 55. The method for constructing a detector as recited inclaim 28 wherein the mold further comprises apertures in the cast. 56.The method for constructing a detector as recited in claim 31 whereinthe malleable airform further comprises apertures in the cast.
 57. Thedetector as recited in claim 1 wherein the array includes at least twodifferent types of photodetectors.
 58. The detector as recited in claim2 wherein the array includes at least two different types ofphotodetectors.
 59. The detector as recited in claim 3 wherein the arrayincludes at least two different types of photodetectors.